US3170791A - Process for making a stencil screen coating and composition therefor - Google Patents

Description

United States Patent 3,170,791 PROCESS FOR MAKING A STENCIL SCREEN COAT- ING AND COMPOSITION THEREFOR I Clarence T. Cunningham, Alton, Ill., assignor to Owens- Illinois Glass Company, a corporation of Ohio N0 Drawing. Filed Feb. 10, 1961, Ser. No. 88,296 12 Claims. (CI. 96-35) This invention relates to a stencil screen and more particularly to a coating for use in filling the voids of a stencil screen whereby portions of the coating are subsequently removed according to photographic methods to form a preselected pattern of voids on the screen. This application is a continuation-in-part of copending application Serial No. 721,337, filed March 14, 1958.

In the production of stencil screens, gelatin has been Widely used as a coating therefor because of its ready availability and low cost. In making such stencil screen compositions, the gelatin is first dissolved in hot water and a light-sensitizing agent and a coloring dye are added thereto. The purpose of the dye is to make the composition more visible so that the ultimate user of the screen can readily perceive which portion of the screen is coated therewith. The viscous gelatin solution is then coated onto the fine mesh screen, either by a squeegee or by immersing the screen while flat in a shallow pan containing some of the composition so that the composition will just penetrate the interstices of the screen so as to surround them and provide a physical bond to the. screen. The coated screen is then dried, for instance, in a mild hot-air oven.

The light-sensitizing agent enables a photographic process to be used in making a stencil of the coated screen. A film in the nature of an opaque positive of the desired decoration or printing is placed over the fine mesh screen coated with the light-sensitized composition, and this assembly is exposed to a light source such as an arc lamp. The arrangement is such that the light issuing therefrom will pass through the negative and contact the coated screen at those areas outlined by the transparent portion of the negative. The light contacted composition is con verted to a state of Water insolubility, whereas the portion of the composition which is not contacted by light remains water soluble.

The coated screen is then given a mild wash With Water to remove the Water soluble portion, and thereby yield the final stencil which bears a coating outlining the desired decoration or printing. The portion of the screen bearing the water-insoluble coating composition has its interstices filled with this composition so that the decorating material, such as printing ink, paint, vitreous enamel, glaze, or color, will not pass thereth-rough.

Stencil screens so produced are conveniently used to apply a color or printing to a substrate such as glassware, e.g., bottles, tumblers, and the like. The stencil held in a frame is brought into contacting relationship with a tumbler or bottle held in a chuck. Decorating medium or color on the screen thus is forced through the unfilled interstices of the stencil screen by a squeegee onto the underlying surface in the desired pattern. After either the stencil or the bottle are retracted so that they are out of contact, the bottle or tumbler is passed into. an oven for baking, in the case of a heat-hardenable decorating material.

Heat, of course, is not always necessary, inasmuch as cold colors have been developed which will harden quickly under ambient conditions. The rigorous service conditions to which the stencils are exposed frequently cause the stencil screen to crack, necessitating the shut-down of the automatic equipment for substitution of new screens therein. Even if no definite cracking occurs, the stencil screens wear out due to gradual degradation of the coating.

g In an effort to enhance the life of these screens, the art has resorted to several modifications. In one embodiment, the screen which has been coated with the gelatin composition in the desired pattern and dried (as described above) is dipped into a glycerin bath. The glycerin adheres to the coated stencil screen, and the screen is then again dried. The glycerin overcoat is found to impart a slight increase in strength to the coating and, consequently, longer life of the stencil in the automatic equipment. Another embodiment which has been utilized involves the inclusion of glycerin in the melted gelatin solution. Glycerin, being water soluble, readily goes into solution with the water containing the melted gelatin. It has been found that this also slightly upgrades the coating composition, thereby increasing the life of the screen formed therefrom Unfortunately, neither of the above embodiments is so advantageous that it really justifies the expense of the modification Thus, the coating of the stencil screen with glycerin necessitates an extra step in the manufacture of the screens, and consequently increases the cost Further, the cost of glycerin itself adds to the expense of the ultimate screen.

Accordingly, it is an object of the present invention to provide an improved stencil screen masking composition which permits the production of a stencil screen possessed of longer life than those known heretofore.

It is another object of this invention to provide such a stencil screen coating composition which is relatively inexpensive, can be produced by a relatively simple process, and yet yields a stencil screen of improved quality.

It is yet another object of this invention to provide a stencil screen composition which, by reason of its greater strength and consequent longer life, materially reduces the down time for the automatic equipment used in applying decorations to a substrate.

It is still another object of this invention to provide a compositionwhich is tougher and less apt to crack under the rigorous service. conditions encountered with auto matic color decorating and printing machines.

The above and many other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and the examples set forth, in which reference Will be had to several preferred embodiments.

Basically, the present invention is predicated upon the discovery that an improved gelatinous stencil screen masking composition may be produced by including in the aqueous solution of gelatin an ester of phthalic acid such as the dialkyl ester wherein the alkyl contains from 2 to 10 carbon atoms.

Example I An improved stencil screen composition of the present invention was prepared according to the following formulation:

The above ingredients were combined in the following manner: cc. of water at a temperature of. 180 F. Was poured into a glue pot. The dye and gelatin were added, and the mixture heated to about F. The

heat was maintained until the gelatin had gone into solution (about 30 minutes). Next, the dibutyl phthalate was added to the solution and the mixture stirred. In a separate container, the sodium bichromate was dissolved in 400 cc. of hot water, and this solution was poured into the gelatin solution and mixed until blended. This took about three minutes. Next, hot water was run through a colloid mill to bring the mill up to the same temperature as the previously prepared mixture, e.g., about 175 F. When the mill was thus warmed, the gelatinous mixture was run through the mill five times in order to more or less homogenize the ingredients. The completely mixed composition was then strained through a 165 mesh stainless steel screen into a shallow pan maintained at about 140 F. It was allowed to set for about five minutes to allow air bubbles to rise to the surface of the liquid. The exposed surface of the liquid was then scraped to remove the bubbles so that the screen could be floated. To float a screen, it is held in a horizontal position and just immersed in the composition. When it is lifted slowly therefrom, the bottom side of the screen will be formed to be more heavily coated than the top side. The composition, of course, does penetrate through the interstices of the screen as described hereinbefore, but, as indicated, only enough is contained on the upper side of the screen to insure its being retained thereby. The coated screen was then dried in a darkroom, after which an opaque positive film of the desired design, and in which the ultimately desired printing or lettering corresponds to the opaque part of the film and the remaining areas thereof are translucent so as to permit transmission of light therethrough, was placed over the screen, and the assembly exposed for about one minute to light issuing from an arc lamp located on the film side of the assembly. The portion of the composition contacted by the light is converted thereby to a cured state which is insoluble in water. The screen was given a mild wash with warm water, whereupon the portion of the coated screen which had not been exposed to light washed away, leaving the stencil screen bearing a water-insoluble coating which outlined the desired decoration, printing, or the like. This stencil screen was found to be extremely tough as compared to the conventional gelatin screens or to the gelatin screens which had been modified by overcoating with glycerin or combining glycerin with the melted gelatin in the formulation thereof. The stencil screen was not subject to cracking, and was capable of undergoing a substantially greater number of cycles in automatic decorating equipment without material degradation.

Example 11 An improved stencil screen coating composition of the invention was prepared according to the same formulation and in the same manner as described in connection with Example I, except that dioctyl phthalate was substituted for dibutyl phthalate. It was found that the composition yielded a stencil screen which was possessed of the same desirable qualities as the stencil produced according to Example I, e.g., longer life, tougher, non-cracking, and, at the same time, relatively inexpensive.

Example III An improved stencil screen coating composition of the invention was prepared according to the same formulation and in the same manner as described in connection with Example I, except that di Carbitol phthalate (the diester of phthalic acid and diethylene glycol monoethyl ether) was substituted for dibutyl phthalate. It was found that the composition yielded a stencil screen which was possessed of similar desirable qualities, e.g., longer life, tougher, non-cracking, and, at the same time, relatively inexpensive.

Example IV Following the aforesaid procedure, 25 grams of solid granular gelatin, technical grade, was dissolved in 100 grams of water. To this was added 1 gram of solid Monastral Blue dye and 4 grams of potassium dichromate light sensitizer. This solution was combined with 25 grams of diethyl phthalate and blended for five minutes The resulting solution was applied to a stencil screen and exposed by an arc lamp. The image produced was clear and sharp, and the resulting stencil screen had the desired qualities of the screens of the aforesaid examples.

Example V Again following the aforesaid procedure, 25 grams of solid granular gelatin, technical grade, was dissolved in 100 grams of water. To this was added 1 gram of solid Monastral Blue dye and 4 grams of potassium dichromate light sensitizer. This solution was combined with 25 grams of di-Z-ethylhexyl phthalate and blended for five minutes. The resulting solution was applied to a stencil screen and exposed by an arc lamp. The image produced was clear and sharp, and the stencil had the properties of the aforesaid screens.

The term phthalic acid ester as used herein is intended to include an ester derived from either phthalic acid or phthalic anhydride, inasmuch as the resulting ester will be essentially the same regardless of which is used in forming it.

The amount of phthalic acid ester used in forming the improved stencil screen coating composition of the invention is preferably about equal to or slightly in excess of the amount of gelatin on a weight basis. However, tests have shown that the gelatin may be present in amounts of from 30 to parts by weight, based upon the combined weight of the ester and the gelatin with the ester then being present in amounts of from 70 to 20 parts by weight.

Exactly why the modification of gelatin by the inclusion of the dialkyl ester of phthalic acid wherein the alkyl has from 2 to 10 carbon atoms, such as diethyl phthalate, dibutyl phthalate, dioctyl phthalate, etc., and di Carbitol phthalate advantageously improves the properties in the manner described is not known. However, and although it is not intended to be bound by any theory expressed herein, it is possible that these materials, although being water-insoluble (whereas glycerin is water-soluble), contribute an internal physical plasticization, and thus lend to the final composition a resiliency which is reflected in the improved service performance of the stencil screens.

Other light-sensitizing agents than sodium bichromate may, of course, be used in forming the masking compositions of this invention. Thus, any of the conventional light-sensitizing agents (essentially oxidizing materials) maybe used, although the dichromates of potassium, sodium, and/or ammonium are preferred.

The composition of this invention may be applied to metallic wire screens or to silk or nylon screens with equal facility. Where applied to wire, the composition may be removed by burning it off with flame, thus permitting the wire screen to be salvaged and reused, if desired. For nylon or silk screens, a solution of bleach such as a hypochlorite is conveniently used to remove the composition.

While there has been disclosed certain embodiments of the invention, it is not thereby desired or intended to be limited solely thereto, for the precise proportions of the materials utilized may be varied and equivalent chemical materials may be employed, if desired, without departing from the spirit and scope of the invention as defined in the appended claims.

Having fully described the invention, what is claimed 1. A stencil screen comprising a screen base and a coating on said screen base, said coating filling the voids of the screen base and wherein subsequently portions of said coating are removed according to photographic methods in order to form a preselected pattern of voids on said screen base, said coating comprising an aqueous mixture of (a) gelatin, (b) a light sensitizing agent to permit photographic method production of the desired pattern by rendering water insoluble that portion of said coating exposed to light, and (c) a dialkyl phthalate wherein the alkyl group contains from 2 to of carbon atoms, the amount of (a) being from 30 to 80 parts by weight based on the combined weight of (a) and (c).

2. A stencil screen as defined in claim 1 wherein the light sensitizing agent is a dichromate light sensitizing agent.

3. A stencil screen as defined in claim 1 wherein the amount of said dialkyl phthalate to said gelatin is substantially equal by weight.

4. A stencil screen as defined in claim 1 wherein is (c) is diethyl phthalate.

5. A stencil screen as defined in claim 1 wherein (c) is dibutyl phthalate.

6. A stencil screen as defined in claim 1 wherein (c) is dioctyl phthalate.

7. A stencil screen as defined in claim 1 wherein (c) is di-Z-ethylhexyl phthalate.

8. A stencil screen comprising a screen base and a coating on said base, said coating filling the voids of the said screen base and wherein subsequently portions of the coating are removed according to photographic methods of production to form a preselected pattern on said screen base, said coating comprising an aqueous mixture of (a) gelatin, ([2) a light sensitizing agent to permit photographic method production of the desired pattern by rendering Water insoluble that portion of said coating exposed to light, and (c) the diester of phthalic acid and diethylene glycol monoethyl ether, the amount of (a) being from 30 to 80 parts by Weight based upon the combined weight of (a) and (c).

9. A stencil screen as defined in claim 8 wherein the light sensitizing agent is a dichromate light sensitizing agent.

10. A stencil screen comprising a screen base and a coating on said base, said coating filling the voids of the said screen base and wherein subsequently portions of the coating are removed according to photographic methods to form a preselected pattern of voids on said screen base, said coating comprising an aqueous mixture of:

G. Water 1000 Gelatin 230 Dibutyl phthal-ate 260 Dye 12 Dichromate light sensitizing agent 20 11. In the method of forming a preselected pattern of voids on a stencil screen comprising the steps of filling the voids of said stencil screen with a coating of an aqueous mixture of gelatin and a light sensitizing agent, dry-ing said coating on said screen, placing an image containing member having transparent and nontransparent portions before said screen, exposing said screen to light passing through said transparent portion of said image containing member, that portion of the gelatin coating in said voids of said screen contacted by said light bein rendered water insoluble, the remainder of said gelatin coating on said screen remaining water soluble, and washing said screen to remove said water soluble portion of said gelatin coating, said voids containing said Water insoluble coating thus defining a pattern of the configuration of the transparent portion of said image containing member, the improvement wherein said aqueous mixture of gelatin and light sensitizing agent contains a dialkyl phthalate wherein the alkyl group has from 2 to 10 carbon atoms, the amount of gelatin in said solution being from 30 to parts by weight based on the combined weight of gelatin and phthalate.

12. In the method of forming a preselected pattern of voids on a stencil screen comprising the steps of filling the voids of said stencil screen with a coating of an aqueous mixture of gelatin and a light sensitizing agent, drying said coating on said screen, placing an image containing member having transparent and nontransparent portions before said screen, exposing said screen to light passing through said transparent portion of said image containing member, that portion of the gelatin coating in the said voids of said screen contacted by said light being rendered water in soluble, the remainder of said gelatin coating on said screen remaining water soluble, and washing said screen to remove said water soluble portion of said gelatin coating, said voids containing said water insoluble coating thus defining a pattern of the configuration of the transparent portion of said image containing member, the improvement wherein said aqueous mixture of gelatin and light sensitizing agent contains a diester of phthalic acid and diethylene glycol monoethyl ether, the amount of gelatin in said solution being from 30 to 80 parts by weight based upon the combined weight of gelatin and diester.

References Cited by the Examiner UNITED STATES PATENTS 2,218,255 10/40 Weyers et a1. 99-111 2,322,027 6/43 Jelley et al. 9697 2,685,510 8/54 Yackel 96-35 X 2,801,171 7/57 Fierke et a1. 9697 2,940,854 6/60 Gray 96-94 2,943,936 7/60 Spechler 9635 X 2,980,534 4/61 Printy et a l. 96-93 X OTHER REFERENCES Kosloff: Screen Process Printing, Signs of the Times Publishing Co. Cincinnati, 1950, pages 91-97.

Doolittle: The Technology of Plasticizers, John Wiley and Sons, New York 1954, pages 951, 952 and 962.

NORMAN G. TORCHIN, Primary Examiner.

MILTON STERMAN, PHILIP E. MANGAN,

Examiners.

Claims (1)

11. IN THE METHOD OF FORMING A PRESELECTED PATTERN OF VOIDS ON A STENCIL SCREEN COMPRISING THE STEPS OF FILLING THE VOIDS OF SAID STENCIL SCREEN WITH A COATING OF AN AQUEOUS MIXTURE OF GELATIN AND A LIGHT SENSITIZING AGENT, DRYING SAID COATING ON SAID SCREEN, PLACING AN IMAGE CONTAINING MEMBER HAVING TRANSPARENT AND NONTRANSPARENT PORTIONS BEFORE SAID SCREEN, EXPOSING SAID SCREEN TO LIGHT PASSING THROUGH SAID TRANSPARENT PORTION OF SAID IMAGE CONTAINING MEMBER, THAT PORTION OF THE GELATIN COATING IN SAID VOIDS OF SAID SCREEN CONTACTED BY SAID LIGHT BEIN RENDERED WATER INSOLUBLE, THE REMAINDER OF SAID GELATIN COATING ON SAID SCREEN REMAINING WATER SOLUBLE, AND WASHING SAID SCREEN TO REMOVE SAID WATER SOLUBLE PORTION OF SAID GELATIN COATING, SAID VOIDS CONTAINING SAID WATER INSOLUBLE COATING THUS DEFINING A PATTERN OF THE CONFIGURATION OF THE TRANSPARENT PORTION OF SAID IMAGE CONTAINING MEMBER, THE IMPROVEMENT WHEREIN SAID AQUEOUS MIXTURE OF GELATIN AND LIGHT SENSITIZING AGENT CONTAINS A DIALKYL PHTHALATE WHEREIN THE ALKYL GROUP HAS FROM 2 TO 10 CARBON ATOMS, THE AMOUNT OF GELATIN IN SAID SOLUTION BEING FROM 30 TO 80 PARTS BY WEIGHT BASED ON THE COMBINED WEIGHT OF GELATIN AND PHTHALATE.